1. Field of the Invention
The present invention relates to a method for molding powder under compression, which is used to mold powder into tablets or the like.
2. Description of the Prior Art
When powders of medicine, food, bubble bath, and the like are to be compressed and molded into tablets, a rotary powder compression molding machine which is called a tablet machine is usually employed. The rotary powder compression molding machine includes a disc-shaped turntable which is horizontally supported and rotatable in a predetermined direction about a vertical axis. The turntable is provided with many bores extending vertically therethrough. The bores are arranged at regular intervals in the direction of rotation of the turntable. A pair of pressure rods are respectively disposed above and below each of the bores in such a manner that they can move toward and away from the bore. These upper and lower pressure rods include compression members which are fitted into the bore as the pressure rods move toward the bore. As the turntable rotates, the upper and lower pressure rods are moved together with the turntable in the direction of its rotation.
With the use of such a compression molding machine, a conventional compression molding process is performed as follows. The compression member of each of the lower pressure rods is always fitted in the corresponding bore, thereby closing the lower opening of the bore. In this state, as the turntable rotates, the lower pressure rods are moved downward while moving in the direction of rotation of the turntable. The downward movement of each pressure rod gives a space in the bore to which powder is supplied. As a result, the bores are successively filled with the powder.
Then, the upper and lower pressure rods are pressed first by upper and lower pre-loading rollers, and then by upper and lower pressure rollers, respectively, toward the corresponding bore, so that the powder is compressed and molded into a tablet. The thus produced tablet is pushed out of the bore by the lower pressure rod. In this way, with each revolution of the turntable, tablets are successively produced.
In such a rotary powder compression molding machine, air is removed from the powder by the compression effected by the pre-loading rollers for the purpose of preventing a problem of blowing out of the powder from the bore during the compression effected by the pressure rollers and a problem of capping and lamination in the resultant tablets. These problems are effectively avoided when the powder of excellent flowability is molded, wherein the air can be sufficiently removed from the powder by the compression effected by the pre-loading rollers.
However, when the powder contains a large amount of wax-like material such as vitamin E which softens in heat or a large amount of liquid which becomes viscous by the application of heat, the above problems arise for the following reason. In the conventional compression molding machine, heat is generated through friction between the inner wall of the bore and the two pressure rods corresponding thereto. Thus, the temperature in each bore increases with time. When the compression molding operation continues for a long period of time, the temperature in each bore becomes so high as to soften the wax-like material contained in the powder or allows the liquid contained in the powder to become viscous. The softened wax-like material or viscous liquid then sticks to the wall of the bore and the pressure rods, thereby narrowing or closing the gaps therebetween. This prohibits the air in the powder from being sufficiently removed from the bore during the compression by the pre-loading rollers, allowing the powder containing air therein to be molded into tablets. The resultant tablets may possibly have disadvantageous capping or lamination.
Furthermore, the air contained in the powder is likely to rapidly blow out through the narrowed gap between the bore and the pressure rods, and, together with the air blown out, a large amount of powder may possibly blow out of the bore during the final compression effected by the pressure rollers. As a result, the amount of the powder to be molded into a tablet is greatly decreased. Even from a gap which is not narrowed, a large amount of powder is likely to be spilled out. The powder blown out or spilled out may enter parts of the machine, causing abnormal wearing of the parts.
The above problems caused by the increase in temperature of the bore can be avoided to some extent by lowering the operation speed of the compression molding machine, so as to reduce heat generated through friction between the bore and the pressure rods. The problems may also be prevented by lowering the compression speed so that air can be sufficiently removed from the powder. However, lowering the operation speed will greatly reduce the production efficiency.